Calcium phosphate and calciumchlorophosphate compositions

ABSTRACT

Novel feed grade calcium phosphate compositions are provided which contain calcium chlorophosphate in varying amounts. This novel composition and other heretofore known varieties of feed grade calcium phosphates are prepared by low temperature drying steps (spray drying or crystallization-evaporation) of solutions or slurries which have been partially defluorinated to the extent that the solution P/F ratio is at least 30-35 but less than 100. The novel compositions described above have adequate levels of phosphorus availability to animals thereby permitting their usage as an animal feed supplement.

This is a continuation of application Ser. No. 170,938, filed Aug. 11,1971, which is a division of Ser. No. 791,944 filed Jan. 17, 1969 whichis now U.S. Pat. No. 3,652,206 (1972).

BACKGROUND OF THE INVENTION

Calcium phosphate is widely used as a feed supplement to animal diets inorder to provide animals with required amounts of phosphorus.Unfortunately, the phosphate rock commonly used to prepare calciumphosphate can contain large quantities of fluorine, as much as 4percent. The presence of this much fluorine in an animal feed is knownto cause fluorosis in the animals and thus it must be removed at somestage during preparation of the calcium phosphate. To be acceptable as a"feed grade" calcium phosphate, the phosphorus to fluorine weight ratio(P/F) in the product must be 100 or greater, a standard hithertoaccepted in the art.

A host of methods for producing defluorined calcium phosphate are known.Methods which produce volatile fluorine compounds are most undesirablebecause of the severe air pollution they can create. Other methods arenot used commercially because of their relatively high cost in relationto an inexpensive product. A preferred defluorination process isinexpensive and ties up the fluorine in liquid or solid form for easyand safe disposal.

U.S. Pat. No. 3,151,936 teaches single step defluorination of acidsolutions of phosphate rock by simply subjecting the solutions tothin-film drying. More specifically, phosphate rock is digested withphosphoric acid and a more volatile mineral acid such as HCl. Thesolution is then spray-dried at high temperatures to yield feed gradecalcium phosphate containing less than 5 percent volatiles andpreferably less than 2 percent volatiles. The gaseous effluent from thedryer is condensed, defluorinated, and then recycled for use inacidulating more rock.

The reference process, it is noted, is directed towards essentiallytotal removal of fluorine during the drying step. To achieve this infeasible commercial equipment such as a spray drier, very hightemperatures i.e. 400°F. or higher, are ordinarily required. Such hightemperatures are known to cause pyrolysis of calcium phosphate topyrophosphates and metaphosphates, a form wherein phosphorus is lessavailable to animals, this not only diminishes product yield but alsoproduces a contaminated product.

When total defluorination is achieved in the drying step, the acidcondensate naturally contains large amounts of fluorine. Recirculationof this acid to the rock digestors, without intervening defluorination,is not feasible since fluorine would soon accumulate in the digestors toa point where satisfactory defluorination in the subsequent drying stepwould no longer be achievable.

A further disadvantage when total defluorination is achieved duringdrying is the danger of fouling the equipment with SiO₂ formed by thehydrolysis of SiF₄.

Applicant has now discovered novel methods which employ spray drying andcrystallization techniques whereby calcium phosphate can be essentiallytotally defluorinated without the use of very high drying temperaturesand all the aforementioned difficulties associated with such hightemperatures.

Moreover, with applicant's process, condensed acid streams from thedrying step can be conveniently recycled, without interveningdefluorination, for use in acidulating rock.

Applicant, in the course of carrying out his process, further producesnovel calcium phosphate compositions which contain varying amounts ofcalcium chlorophosphate but which, nevertheless, have an availablephosphorus content comparable to commercially available feed gradecalcium phosphates thereby making such compositions eminently useful asanimal feed supplements.

These and other advantages of this invention will be apparent from thefollowing disclosure.

SUMMARY OF THE INVENTION Spray-Dried Product and Process

This invention relates to a process for preparing mixtures of calciumphosphate and calcium chlorophosphate having a P/F ratio of 100 orgreater by employing a spray drying technique. More particularly, thisinvention envisions preparing such feed grade calcium phosphatecompositions by spray-drying:

a. a solution comprising water, phosphoric acid, and a salt selectedfrom the group consisting of calcium dichloride or calcium nitrate, saidsolution further characterized in having a solution P/F ratio of atleast about 30 but less than 100 and a Ca/P mole ratio which ranges fromgreater than 0.5 to about 1.0. or

b. a slurry comprising solution as defined in (a) and calcium sulfatesolids,

wherein the exit gas temperature from the spray dryer is maintained atabout 200°F. to about 380°F., and recovering the solid product from thedryer.

In cases where the calcium and phosphate in the above described solutionoriginate directly from acidulated phosphate rock, the solution mayalternatively contain an acid selected from the group consisting ofhydrochloric and nitric, the anion of said acid corresponding to theanion of the salt contained in the solution.

The solutions contemplated by this invention include those which areproduced merely by mixing aqueous solutions of calcium dichloride orcalcium nitrate with phosphoric acid as well as those which are preparedin situ by acidulating phosphate rock. For purposes of clarity, however,the invention is described primarily in terms of solutions prepared byacidulation of phosphate rock.

Solutions or slurries as described above can be prepared by knownmethods as will be further discussed hereinbelow. They can also beprepared by a novel method which comprises dissolving phosphate rock insulfuric acid and either HCl or HNO₃, adding to the solution a solublesodium or potassium salt, filtering to remove solids, cooling thefiltrate to cause additional precipitation of solids, removing theinsolubles, and then, if necessary to achieve the required mole ratio ofCa/P in solution, adding more H₂ SO₄ to the filtrate to produce theslurry described above. If sufficient H₂ SO₄ is added in the digestor toachieve the desired Ca/P mole ratio, addition of more H₂ SO₄ is notrequired thereby eliminating formation of the slurry and yielding anacidic solution as described above. Preparation of the solution orslurry by said novel process will be discussed in greater detailhereinbelow.

Due to the comparatively high P/F ratio (i.e. low fluorine content) inthe solution prior to drying, the gaseous effluent from the spray dryerwill not contain undue amounts of fluorine. Hence this effluent can becondensed or otherwise converted to liquid form and recycled, without anintervening defluorination step, to the digestors for use in acidulatingmore rock. Any small accumulation of fluorine which might tend to occurwould be eliminated in the defluorination steps to which the solution issubjected prior to spray drying in order to raise the P/F ratio to atleast 30 before drying.

This invention also relates to the novel calcium phosphate compositionsprepared by the aforementioned process. In particular, this compositioncomprises calcium phosphate and calcium chlorophosphate, saidcomposition further characterized by a P/F ratio of 100 or greater.

The amount of calcium chlorophosphate which appears in product preparedby the process of this invention will depend upon the solution of Ca/Pratio in the solution or slurry fed to the spray dryer with higher ratiovalues favoring the formation of more calcium chlorophosphate in theproduct. For example, when the Ca/P ratio is only slightly higher than0.5, the product will contain only small amounts (i.e. 1 percent) ofcalcium chlorophosphate; on the other hand, if the Ca/P ratio is 1.0, itis possible under certain drying conditions to produce substantiallypure (i.e. 100 percent) calcium chlorophosphate. A Ca/P mole ratio of0.59 in the solution fed to the spray dryer produces a productcontaining 25 percent calcium chlorophosphate, this product having achloride content of 5 percent.

The inclusion in the mixture of major amounts of calcium chlorophosphatewithout adverse effect on the amount of phosphorus available to animalsis most surprising since heretofore it had been believed that thephosphorus content of calcium chlorophosphate was unavailable toanimals.

The discovery that calcium chlorophosphate is suitable for use as ananimal feed supplement is also related to the comparatively low dryingtemperatures of the process of this invention. This is so because it wasbelieved heretofore that as much chloride as possible had to be removedfrom the product. Moreover, it was appreciated that to remove suchchloride, high drying temperatures were necessary with all thedisadvantages of high temperture drying as already discussed. Byeliminating the need for removal of chloride from the product and byproviding a novel method of removal of fluorine from the product undermild process conditions, a convenient process is made available forproducing a high quality animal feed supplement.

The term "calcium phosphate" as used herein can mean either monocalciumphosphate, dicalcium phosphate, or hydrated forms of the prior twophosphates, as well as mixtures of two or more of the aforementionedphosphates or hydrates thereof.

This invention further relates to a method for supplementing thephosphorus content in an animal's diet which comprsises administering tothe animals by oral ingestion a biologically acceptable amount of theafore-described novel composition or substantially pure calciumchlorophosphate.

CRYSTALLIZATION PROCESS

This invention also relates to a process for preparing crystallinemonocalcium phosphate and hydrated forms thereof having a P/F ratio of100 or greater by the use of a solution evaporation technique. Moreparticularly, this invention envisions preparing such feed grademonocalcium phosphate by feeding to a crystallizer or other suitableapparatus containing therein phosphoric acid:

a. a solution comprising water and a salt selected from the groupconsisting of calcium dichloride or calcium nitrate, and

b. phosphoric acid

The solution and phosphoric acid can be fed to the crystallizer eitheras separate streams or they may be combined into a single stream priorto entering the crystallizer. The feed to the crystallizer is furthercharacterized in having a solution P/F ratio of at least about 35 and aCa/P mole ratio of about 0.5, said P/F and Ca/P ratios being basedsolely on the P, F, and Ca contained in the solution and phosphoric acidfed to the crystallizer. The mixture in the crystallizer is then heatedor otherwise treated to remove water and other volatiles wherebycrystals of monocalcium phosphate or hydrated forms thereof areproduced. When this process is operated continuously at equilibrium,crystallizer mother liquor is continuously withdrawn from thecrystallizer along with the monocalcium phosphate crystals. This motherliquor is separated from the crystals and recycled to the crystallizerwhile the crystals themselves are recovered.

The solution described above may also contain an acid selected from thegroup consisting of hydrochloric or nitric, the anion of said acidcorresponding to the anion of the salt in the solution. In the event thesolution does contain such an acid, it is desirable to maintain the P₂O₅ content in the crystallizer slurry between 35 percent and 60 percent.In computing these P₂ O₅ percentages, both soluble and insoluble P₂ O₅in the slurry is included. Maintenance of these P₂ O₅ contents in theslurry permits the acid strength of the gaseous effluent stream from thecrystallizer to be kept within limits which allow said stream to berecycled for use in acidulating additional phosphate rock as isdiscussed in greater detail hereinbelow.

A suitable acidic crystallizer feed solution can be prepared in the samemanner as described heretofore for the feed to the spray dryer.Similarly, the gaseous effluent from the crystallizer can be condensedand recycled, without intervening defluorination, to the digestors foruse in acidulating phosphate rock.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowsheet showing a preferred embodiment of aspray-drying defluorination process and a crystallization process inaccordance with the process of this invention.

FIG. 2 is a plot of P/F ratio in the spray dried product vs. the spraydryer and is provided to indicate the criticality of the P/F ratio inthe solution or slurry being at least about 30.

FIG. 3 is a plot of the P₂ O₅ and chloride analyses of the spray dryedproduct vs. the exit gas temperature of the spray dryer and is providedto indicate the flexibility of product composition which is obtained byvarying the exit temperature with the proscribed limits of thisinvention.

FIG. 4 is a plot of equilibrium data which defines the amount of P₂ O₅in the crystallizer slurry at various conditions of pressure andtemperature within the crystallizer.

FIG. 5 is a plot of the amount of hydrochloric acid in the crystallizerslurry and in the recycle condensate vs. the amount of P₂ O₅ in thecrystallizer slurry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before a solution or slurry can be spray dryed or evaporated to yield afeed grade product, it is usually necessary to remove at least a portionof the fluorine originally present in the solution in order to establishthe required solution P/F ratio of at least 30 or 35. In the case wherethe solution is prepared in situ by acidulation of phosphate rock, it isalways necessary to remove some of the fluorine originally present inthe rock in order to establish the required solution P/F ratio of atleast 30 or 35.

Such partially defluorinated solutions can be prepared by heretoforeknown techniques. For example, one known method is to treat the solutionwith hydrofluoric acid in order to achieve an optimum F/Al mole ratio insolution of about 14-16 which assures maximum precipitation of analuminum fluorine complex whose probable formula is presentedhereinbelow. Precipitation of this complex not only removes the fluorineadded as hydrofluoric acid but also up to 90 percent of the fluorineoriginally present in the rock. U.S. Pat. No. 2,976,141 exemplifies thehydrofluoric acid technique. This technique by itself cannot producesolution P/F ratios of 100 or greater but will produce ratios of 30 orgreater. The hydrofluoric acid technique, however, is not preferred dueto the expense and highly corrosive effect of the acid.

It is also possible to partially defluorinate such solutions by additionof SiO₂ to the solution. The fluorine is then volatilized as SiF₄. Thisis not desirable, however, because of possible fouling of metalequipment and heat transfer surfaces with SiO₂ at subsequent stages inthe defluorination process.

A preferred process which incorporates a novel partial defluorinationprocess prior to the spray drying and crystallization steps is presentedschematically in FIG. 1. The preparation of solution suitable for spraydrying or evaporation is essentially the same. Referring to FIG. 1,phosphate rock 11, aqueous sulfuric acid 12, and aqueous hydrochloricacid 13 are fed to a first digestor 14 wherein the rock is digested toyield a slurry 15 comprising (a) a solution of water, hydrochloric acid,calcium chloride, phosphoric acid, and soluble fluorine present as avariety of soluble fluorine containing compounds, and (b) a solidportion made up of calcium sulfate, a variety of insolubles from therock such as SiO₂, and an aluminum fluorine complex having the probableformula:

    CaSO.sub.4.CaF.sub.2.AlF.sub.3.1.5 CaSiF.sub.6.12 H.sub.2 O

sufficient total acidity must be provided by the sulfuric andhydrochloric acid to dissolve the phosphate rock. Preferably a 20 to 30percent excess of total acidity is used. The rock itself can be anycommercial phosphate rock which is essentially fluorapatite such asFlorida, North Carolina, or Moroccan rocks, also western phosphate rocksand igneous phosphates such as Quebec and Kola rock. The P/F ratio inmost rocks is approximately 3.5. Of course, nitric acid can be used inplace of the hydrochloric acid but hydrochloric acid is highly preferreddue to its low cost and availability.

The ratio of hydrochloric to sulfuric acid employed in digestor 14depends upon the nature of the product which is to be made. The functionof the H₂ SO₄ is to remove sufficient calcium from solution as calciumsulfate to establish the desired mole ratio of Ca to P in solution. Ifmonocalcium phosphate crystals are to be prepared in the crystallizationprocess, it is desirable to use enough H₂ SO₄ to establish a Ca/P moleration in solution of 0.4--0.5. If the spray drying route is to befollowed to prepare a mixture of calcium phosphate and calciumchlorophosphate, sufficient H₂ SO₄ should be used to establish a Ca/Pmole ratio in solution of > 0.5-1.0. However, since the feed to thespray dryer may be either a solution or a slurry, there are severalpossible stages in the process where the H₂ SO₄ can be convenientlyadded. For example, if a solution is to be spray dried, all of therequired H₂ SO₄ is added during the initial digestion of the rock.However, where flexibility in the phosphorus content of the productemerging from the spray drier is desired, it becomes convenient to addonly a portion of the stoichiometric amount of H₂ SO₄ required in theinitial digestion step, adding the rest of it to the solutionimmediately prior to spray drying. In this case, calcium sulfate appearsin the product as diluent. It can be seen that by varying the amount ofthe H₂ SO₄ added to the solution immediately prior to spray drying, thepercentage of phosphorus in the product can be varied by the appearanceof more or less calcium sulfate diluent in the product.

There will be some formation of the aforementioned insoluble aluminumfluorine complex in digestor 14 causing removal of about 35 to 50% ofthe fluorine originally present in the rock. As a result, the P/F ratioin the solution portion of slurry 15 is usually about 7 to 10. Theamount of fluorine removed as complex fluoride depends somewhat on theAl⁺ ⁺ ⁺ ion concentration in the solution. Excessive amounts of Al⁺ ⁺ ⁺ions in the solution tend to reduce the formation of complex fluoridecrystals. However, in all cases, sufficient fluorine can be complexed toproduce the required solution P/F ratio in the feed to the spray dryeror crystallizer.

Slurry 15 is transferred to a second digestor 16. A water soluble sodiumor potassium salt such as sodium chloride, sodium carbonate, sodiumnitrate, potassium chloride, potassium nitrate, or such is added toslurry 15 in digestor 16 whereupon the salt reacts with the solublefluorine to form the compound Na₂ SiF₆ or K₂ SiF₆, which thenprecipitates from solution removing additional fluorine. The salt isusually added in at least the stoichiometric amount required forreaction with the available fluorine. The precipitation of Na₂ SiF₆ orK₂ SiF₆ will usually remove an additional 10 to 25 percent of thefluorine originally present in the rock resulting in a P/F ratio ofabout 10 to 15 in the solution portion of slurry 17.

The sodium or potassium salt can be added to digestor 15 in lieu ofdigestor 16. It is preferred, however, that it be added at a pointsubsequent to precipitation of the aluminum fluorine complex since itappears that the more aluminum which exists in solution when the salt isadded, the less Na₂ SiF₆ or K₂ SiF₆ which is formed. Therefore, by notadding salt until digestor 16, at which point a large portion ofaluminum has been removed from solution, a somewhat larger crop of Na₂SiF₆ or K₂ SiF₆ is obtainable, resulting in more efficientdefluorination.

Slurry 17 is filtered 18 to remove the various insolubles 19 previouslydiscussed. The filter cake is then advantageously washed with water toproduce wash liquor 18a which is utilized at a later point in thespray-drying process. Filtrate 20 is transferred to clarifier 21 whereit is cooled to further precipitate insolubles. Filtrate 20 isordinarily cooled to a temperature of 10° to 30°F. Solution 22 isseparated from insolubles 23. The clarification step can just asconveniently be performed prior to filtration step 18. Clarificationwill remove about 15 to 25 percent more of the fluorine originallypresent in the rock due to additional precipitation of the aluminumfluorine complex and Na₂ SiF₆. Consequently, the P/F ratio in solution22 will usually be at least about 35, a sufficiently high value topermit solution 22 to be spray dried or crystallized in accordance withthe processes of this invention provided the proper Ca/P mole ratioexists in solution 22.

At this point it becomes convenient to separately discuss subsequentprocessing of solution 22 depending on whether it is to be crystallizedor spray dried.

SPRAY DRYING

Solution 22 can be fed directly to spray dryer 24 (as shown by thedotted line in FIG. 1) provided sufficient H₂ SO₄ was previously addedin digestor 15 to establish a Ca/P mole ratio of > 0.5 to 1.0 insolution 22. However, if solution 22 is to be spray dried, it isadvantageously preconcentrated in evaporator 25, either at ambientpressure or under vacuum to produce concentrated solution 26 which canthen be fed directly to dryer 24 as shown in FIG. 1. On the other hand,if the election has been made to dilute the product with calcium sulfateas heretofore discussed, concentrated solution 26 will not contain therequired Ca/P mole ratio since not all of the H₂ SO₄ required toestablish this ratio was added in digestor 15. In this event, solution26 is transferred to mixing vessel 27 where the remainder of the H₂ SO₄required to establish a Ca/P mole ratio in solution of > 0.5 to 1.0 isadded to solution 26 to produce slurry 28 containing calcium sulfatesolids. Slurry 28 is then fed to spray dryer 24.

If the solution Ca/P mole ratio exceeds 1.0 it becomes difficult toremove the chlorine in the spray drying step unless very hightemperatures are used. If the Ca/P ratio is below 0.5 the dried productwill be undesirably contaminated with phosphoric acid and furthermorewill be substantially free from calcium chlorophosphate.

The spray dryer conditions are controlled by maintaining the temperatureof the gaseous effluent 29 from the dryer at prescribed temperatureswithin a range of 200°F. to 380°F. with preferred exit gas temperaturebeing 200°F. to 350°F. These relatively low temperatures minimizedegradation of the calcium phosphates to undesirable metaphosphates andpyrophosphates and, furthermore, prevent formation of compounds such asSiO₂ which can foul metallic equipment.

FIG. 2 clearly demonstrates that it is essential that the solution P/Fratio in either the solution or slurry fed to the spray dryer be atleast 30 or higher if a dryed product having a P/F ratio of 100 or moreis to be obtained. FIG. 1 was prepared using a variety of data pointsobtained at various dryer exit gas temperatures between 200°-380°F. Thethickness of the plot reflects the slight variations observed withwidely varying exit gas temperatures. However it is apparent that asolution P/F ratio of 30 or higher will produce a spray dried producthaving a P/F ratio of 100 or more under all the process conditions ofthis invention.

The composition of the dry product 30 produced will depend primarily onthe particular value of the Ca/P mole ratio in the solution dried andthe temperature of dryer exit gas 29. FIG. 3 shows the effect of exitgas temperature upon composition of the dry product for a Ca/P solutionmole ratio of about 0.8 and solution feed rates to the dryer of 100 to200 ml./min. At temperatures between 200°-300°F., the composition of theproduct remained fairly constant, the product containing about 60percentmonocalcium phosphate monohydrate and 40 percent calciumchlorophosphate. At temperatures between 300°-380°F., the amount of P₂O₅ in the product rises sharply while the amount of chlorine dropssharply. The product produced in the region between 300°-380°F. is amixture of dicalcium phosphate, monocalcium phosphate, and calciumchlorophosphate with the product formed at 360°F. containing6.66percent, 59.0percent and 32.15percent, respectively, of eachingredient. Curves similar to FIG. 3 can, of course, be readilyconstructed for Ca/P mole ratios other than 0.8 in order to arrive atproper conditions for making a product of any desired composition.

Calcium chlorophosphate can be prepared in high purity using a Ca/P moleratio in solution of 1.0 and exit gas temperatures of 200°-300°F.Monocalcium phosphate can be prepared in high purity using a Ca/P moleratio in solution approximating 0.5 and higher exit gas temperatures(i.e. 350°-380°F.). Dicalcium phosphate is difficult to make in highpurity under any conditions and this particular phosphate will usuallybe made in a mixture with monocalcium phosphate and/or calciumchlorophosphate. It is preferable to leave some chloride in the productmixture to avoid the use of the high drying temperatures required toremove chloride.

Referring to FIG. 1, gas stream 29 can be condensed to yield an aqueousstream of hydrochloric acid and soluble fluorine. The amount of acid instream 29 will, of course, depend on the nature of the product to bemade. Obviously, products containing more calcium chlorophosphate willreduce the amount of acid in stream 29. However, once the desiredproduct composition is known, the acid content is readily computable.The fluorine content in stream 29 will be quite small, usually amountingto about 0.01 to 0.1 percent.

Since gas stream 29 contains non-condensables as well, it isadvantageous to convert the water soluble condensable fraction of stream29 to an aqueous HCl solution 31 for recycle by contacting stream 29with a mixture of the condensed gaseous effluent 32 of evaporator 25 andwash liquor 18a from filtration step 18. Stream 31 can then be recycledto digestor 15 for use in acidulating rock or to reservoir 13. Sincesome hydrochloric acid will usually be consumed as product 30, it willbe necessary in a continuous process to add make-up hydrochloric acid 33from time to time to the system.

The mixtures of calcium phosphate and calcium chlorophosphate preparedby the spray-drying process of this invention have a level of phosphorusavailability to animals which is comparable to commercially availablefeed grade calcium phosphates. This property, coupled with theacceptable P/F ratio, makes these mixtures suitable for use insupplementing the phosphorus requirements of a variety of animalsincluding commercial livestock such as cattle, pigs, and poultry. Themixture is typically admixed with the diet fed to the animals inbiologically acceptable amounts, said amounts being well established inthe art.

It must be understood that for purposes of clarity a particularprocessing sequence was presented in FIG. 1. It is not intended that theprocesses of FIG. 1 represent inflexible process routes since it isreadily apparent to those skilled in the art that a number of differentarrangements and permutations of process steps could be used withacceptable results. Such varied arrangements and permutations, ofcourse, fall within the scope of the claims appended hereto.

CRYSTALLIZATION

Referring again to FIG. 1, solution 22 is fed directly to crystallizer34, the crystallizer containing therein phosphoric acid. The Ca/Psolution mole ratio in solution 22 must not be greater than 0.5,otherwise there will be a buildup of calcium in crystallizer 34 with anattendant loss of acid available for recycle. A Ca/P mole ratio somewhatlower than 0.5, i.e. 0.4-0.5, will cause build-up of phosphoric acid incrystallizer 34. However, such a lower Ca/P ratio would be acceptableprovided crystallizer 34 is purged from time to time to preventphosphoric acid build-up.

The proper Ca/P mole ratio in solution 22 is preferably established byaddition of the required amount of H₂ SO₄ to digestor 15. It is mostundesirable for solution 22 to contain solids such as, for example,calcium sulfate since it will make the product difficult to filter andwill, additionally, defeat the objective of a pure high gradecrystalline monocalcium phosphate.

The solution P/F ratio in solution 22 must be at least about 35 orgreater. This is somewhat higher than the lower range of P/F ratio whichis permissible in the spray dryer feed. However, the defluorinationsteps described above are suitable for partially defluorinating solution22 to provide solution P/F ratios of 35 or greater.

FIG. 4 presents equilibrium data which indicates the conditions ofpressure and temperature within the crystallizer which are required toestablish a given percentage of P₂ O₅ in the crystallizer slurry.

FIG. 5 presents equilibrium data which indicates the amount of HCl inboth the crystallizer slurry and in the condensed effluent gas stream 35from crystallizer 34 as a function of the amount of P₂ O₅ in thecrystallizer slurry. Of particular interest, especially when acontinuous process is to be used, is the acid strength of stream 35.This acid strength can be fixed at any of a variety of concentrations,by simply maintaining the required amount of P₂ O₅ in the crystallizerslurry (as obtained from FIG. 5). This required amount of P₂ O₅ isobtained by merely selecting the required pressure and temperaturewithin the crystallizer (as obtained from FIG. 4).

The strength of the recovered hydrochloric acid stream 35 dependsprimarily upon the concentration of hydrochloric acid in feed solution22. For example, if solution 22 contained 10 percent HCl and 40 percentwater, the strength of recovered acid stream 35 would under equilibriumconditions be about 18-20 percent if minor loss of water as water ofcrystallization in monocalcium phosphate is taken into consideration. Toobtain an acid strength of 18-20 percent in stream 35, the P₂ O₅ contentin the slurry of crystallizer 34 must be, according to FIG. 5, somewherebetween 40 and 45 percent.

When the process is operated continuously under equilibrium conditions,the amount of P₂ O₅ in the crystallizer slurry is preferably maintainedat 35 to 60 percent, corresponding to an evaporator condensate 35containing from about 5 to 25 percent HCl. If the HCl content of stream35 is less than about 5 percent, the acid is too weak for use in thefurther acidulation of phosphate rock. If the HCl content of stream 35exceeds about 25 percent, it is too concentrated to adequatelysolubilize the phosphate rock. Plots similar to FIGS. 4 and 5 can, ofcourse, be readily prepared for a nitric acid system.

The gaseous effluent 36 from crystallizer 34 contains water,hydrochloric acid, and soluble compounds of fluorine in small amounts.Stream 36 typically contains 0.05 to 0.3 percent fluorine. Stream 36 iscondensed to give an aqueous acid solution 35 which is then recycled foruse in acidulating additional phosphate rock in digestor 15.

When the crystallization process is operated continuously atequilibrium, a stream 37 of mother liquor mixed with monocalciumphosphate crystals is continuously withdrawn from crystallizer 34.Stream 37 is filtered 38 or centrifuged to separate mother liquor 39from the crystalline monocalcium phosphate product 40 whereupon motherliquor 39 is advantageously recycled to crystallizer 34. The motherliquor comprises water, phosphoric acid, calcium chloride (or calciumnitrate), monocalcium phosphate, hydrochloric acid (or nitric acid), andimpurities derived from phosphate rock. The concentration of theseingredients depends, of course, on the crystallizer operating conditionssuch as temperature, pressure, feed solution composition, the rate offeed introduction into the crystallizer, recycle rate, and such.

The absolute pressure in the crystallizer can range from 1 cm. of Hg. toatmospheric pressure or higher. However, low pressures are preferredsince this permits the use of lower operating temperatures.

The type of monocalcium phosphate crystallized, i.e. either anhydrous orthe monohydrate, depends upon the temperature and P₂ O₅ concentration ofthe mother liquor and is readily obtainable from a CaO-P₂ O₅ -HCl-H₂ Ophase diagram.

The following examples are provided to further illustrate the invention.

EXAMPLE 1 Preparation of Partially Defluorinated Solutions for use as aFeedstock to the Spray-Dryer or Crystallizer

a. by the process of this invention

50 parts of phosphate rock were digested with 62.4 parts of 21.5 percentaqueous HCl and 29.7 parts of 96 percent aqueous H₂ SO₄. After a 2 hourdigestion period, 2 parts of NaCl were added to the digestion mixtureand digestion continued for an additional 2 hours. The slurry producedwas filtered with the filtrate analysis being as follows:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                             16.64%                                                        CaO:             8.57%                                                        Cl:             16.66%                                                        F:               .28%                                                         ______________________________________                                    

The Ca/P mole ratio of the filtrate was about 0.65 and the P/F ratioabout 26, corresponding to removal of 86 percent of the fluorineinitially present in the rock.

The filtrate was then cooled to a temperature of 10°C. whereupon someinsolubles precipitated from solution. The resulting mixture wasfiltered to yield a filtrate containing 0.12 percent fluorine and havinga P/F ratio of about 60 and a Ca/P mole ratio of 0.65. The P/F ratio of60 indicated that 93 percent of the fluorine initially present in therock was removed.

b. by a prior art process

230 parts of primary phosphate rock was digested with 458 parts of 24.4percent aqueous HCl, 65.6 parts of 96 percent aqueous H₂ SO₄, and 16.2parts of 48.9 percent aqueous HF. The F/Al mole ratio in the digestionmixture was 14. After a 16 hour digestion period, the slurry wasfiltered. The filtrate analyzed as follows:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                             12.1%                                                         CaO:            10.4%                                                         Cl:             18.9%                                                         F:               .087%                                                        ______________________________________                                    

The Ca/P mole ratio in the filtrate was 1.08 and the P/F ratio was 61,corresponding to removal of 93 percent of the fluorine initially presentin the rock.

EXAMPLES 2 TO 5 Preparation of Feed Grade Calcium Phosphate bySpray-Drying of a Partially Defluorinated Solution

The solution prepared in Example 1(a) was dried in a conventional spraydryer at various feed rates and dryer exit gas temperatures. Results aretabulated below and indicate that products having P/F ratios of 100 orgreater were produced under all conditions studied.

    __________________________________________________________________________    Example                                                                            Dryer Temp. (F.)                                                                         Feed Rate                                                                            Product            Product                             No.  Inlet gas                                                                           Exit gas                                                                           (ml./min.)                                                                           % P.sub.2 O.sub.5                                                                  % Cl % F  P/F Composition                         __________________________________________________________________________    2    850   360  100    50.03                                                                               5.99                                                                              0.22 100 Ca(H.sub.2 PO.sub.4).sub.2                                                    CaClH.sub.2 PO.sub.4.H.sub.2 O                                                CaHPO.sub.4                         3    600   305  50     45.91                                                                              10.35                                                                              0.17 118 CaClH.sub.2 PO.sub.4.H.sub.2 O                                                Ca(H.sub.2 PO.sub.4).sub.2          4    455   245  45     44.80                                                                              10.88                                                                              0.164                                                                              119 CaClH.sub.2 PO.sub.4.H.sub.2 O                                                Ca(H.sub.2 PO.sub.4).sub.2          5    350   200  40     44.62                                                                              10.14                                                                              0.168                                                                              116 CaClH.sub. 2 PO.sub.4.H.sub.2                                                 O                                                                             Ca(H.sub.2 PO.sub.4).sub.2          __________________________________________________________________________

EXAMPLE 6 Preparation of Feed Grade Calcium Phosphate by Spray-Drying ofa Partially Defluorinated Slurry

94.5 parts of the solution prepared in Example 1(b) was reacted with 4.6parts of 96 percent aqueous H₂ SO₄. The CaSO₄.2H₂ O thus formed was notremoved from the solution. The mole ratio of Ca/P in the solutionportion of the slurry was about 0.825. The slurry was thenpreconcentrated in a vacuum evaporator before spray drying. The gaseouseffluent from this evaporator was condensed to produce a weak aqueousHCl solution which was subsequently used along with filter cake washingsto absorb the HC1 and other water soluble components of the gaseouseffluent of the spray dryer so that the HC1 might be recycled for use inacidulating more rock.

The concentrated slurry was then spray dried in conventional equipmentwith the temperature of the dryer exit gases being maintained at 360°F.The slurry feed rate to the dryer was about 150 ml./min. The gaseouseffluent from the dryer gas circulated through the evaporator condensateto recover the HC1 as an aqueous solution. The product from the dryeranalyzed as follows:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                             37.2%                                                         F:               .134%                                                        Cl:              6.0%                                                         ______________________________________                                    

corresponding to a P/F ratio of 121. The product was a mixture ofCaHPO₄, Ca(H₂ PO₄)₂, CaClH.sub. 2 PO₄, and CaSO₄.

Recovery of HC1 was 95%. The F/HC1 weight ratio in the recovered HClstream was 0.0023.

EXAMPLE 7 Preparation of feed Grade Monocalcium Phosphate Crystals byCrystallization from a Partially Defluorinated Solution

A partially defluorinated calcium nitrate-phosphoric acid solution wasprepared by digesting phosphate rock with mixed nitric and sulfuricacids by a method similar to that of Example 1(a). The filtered solutionanalyzed:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                             16.5%                                                         CaO:             6.52%                                                        NO.sub.3 :      19.80%                                                        F:               .199%                                                        ______________________________________                                    

The P/F ratio of the solution was about 36 indicating that 89 percent ofthe fluorine initially present in the rock was removed by the formationof the insoluble aluminum fluourine complex and Na₂ SiF₆.

The solution was fed to a crystallizer containing a mother-liquorsaturated with respect to Ca(H₂ PO₄)₂ and having the following analysis:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                              56.16%                                                       NO.sub.3 :       5.08%                                                        F:               .24%                                                         ______________________________________                                    

The crystallizer was operated at 125°C. and 6 cm. Hg. absolute pressure.The solution fed to the crystallizer had a Ca/P mole ratio of about 0.5.In the crystallizer the feed solution was converted into two phases: (1)a solid phase containing Ca(H₂ PO₄)₂ and (2) a vapor phase containingHNO₃, H₂ O and small amount of fluorine as SiF₄ and/or HF. The vaporphase was condensed for recycle to the rock digestion system.Crystalline monocalcium phosphate was separated by filtration of themother-liquor. The product analyzed as follows before washing:

    ______________________________________                                        P.sub.2 O.sub.5 :                                                                              55.2%                                                        NO.sub.3 :       3.96%                                                        F:               .037%                                                        ______________________________________                                    

corresponding to a P/F ratio of 650.

EXAMPLE 8 Preparation of Feed Grade Calcium Phosphate by Spray Drying anAcid Free-Partially Defluorinated Solution

An aqueous solution of calcium dichloride was mixed with phosphoric acidof high fluorine content to produce a solution having a Ca/P mole ratioof 0.8 and a solution P/F ratio of about 20. The solution was treatedwith NaCl and chilled to produce a precipitate in accordance with theprocedure of Example 1(a). The slurry was filtered yielding a filtratehaving a Ca/P mole ratio of 0.8 and a P/F ratio of about 60. Thesolution was then fed to a conventional spray dryer at a rate of about125 ml./min., with the exit gas temperature of the dryer beingmaintained at about 320°F. The dried product had a P/F ratio in excessof 100 and comprised a mixture of CaHPO₄, CaClH₂ PO₄.H₂ O, and Ca(H₂PO₄)₂.

EXAMPLE 9 Preparation of Feed Grade Monocalcium Phosphate Crystals byCrystallization from an Acid Free-Partially Defluorinated Solution

Phosphoric acid of high fluorine content was treated with NaC1 inaccordance with the procedure of Example 1(a) to yield a defluorinatedphosphoric acid having a P/F ratio of about 65. The phosphoric acid andan aqueous solution of calcium dichloride were separately fed to acrystallizer containing therein phosphoric acid. The Ca/P mole ratio inthe feed to the crystallizer was about 0.5 and the P/F ratio about 65(based on the Ca, P, and F in the two feed streams only). Thecrystallizer was operated at 125°C. and 6 cm. Hg. absolute pressure.Crystalline monocalcium phosphate was recovered from the crystallizerhaving a P/F ratio in excess of 100.

In any of the prior examples, hydrochloric and nitric acid can, ofcourse, be used interchangeably.

I claim:
 1. A process for preparing a mixture of calcium phosphate and calcium chlorophosphate having a P/F ratio of 100 or greater which comprises:spray-drying a solution comprising water, phosphoric acid, and calcium dichloride salt, the solution further characterized in having a solution P/F ratio of at least about 30 but less than 100 and a Ca/P mole ratio of which ranges from greater than 0.5 to about 1.0, the exit gas temperature of the spray dryer being from about 200°F. to about 380°F., and recovering the solid product therefrom.
 2. The process of claim 1 wherein said solution further includes hydrochloric acid.
 3. The process of claim 2 wherein the solution is prepared by the steps of:a. digesting phosphate rock with an aqueous solution of sulfuric acid and hydrochloric acid, b. contacting the solution with a water soluble sodium or potassium salt to form the compound Na₂ SiF₆ or K₂ SiF₆, respectively, c. collecting the filtrate, d. cooling the filtrate to cause further precipitation of insolubles and then separating the insolubles from the filtrate, e. adding additional sulfuric acid to the filtrate but only if necessary to adjust the Ca/P mole ratio in solution to from greater than 0.5 to about 1.0.
 4. The process of claim 3 wherein the dryer exit gas temperature is from about 200°F to about 350°F and said solution is preconcentrated prior to being spray dried and further including the steps of condensing the water soluble fraction of the gaseous effluent from said spray dryer or otherwise forming an aqueous solution of said water soluble fraction, and then recycling said condensate or solution, without removal of fluorine, for use in acidulating phosphate rock.
 5. A process for preparing a mixture of calcium phosphate and calcium chlorophosphate having a P/F ratio of 100 or greater which comprises:spray-drying a slurry comprising (a) solution comprising water, phosphoric acid, and calcium dichloride salt, the solution further characterized in having a solution P/F ratio of at least about 30 but less than 100 and a Ca/P mole ratio of which ranges from greater than 0.5 to about 1.0 and (b) calcium sulfate solids wherein the exit gas temperatures of the spray dryer are from about 200°F. to about 380°F., and recovering the solid product therefrom.
 6. The process of claim 5 wherein the solution further includes hydrochloric acid.
 7. The process of claim 6 wherein the slurry is prepared by the steps of:a. digesting phosphate rock with an aqueous solution of sulfuric acid and hydrochloric acid, b. contacting the solution prepared in (a) with a water soluble sodium or potassium salt to form the compound Na₂ SiF₆ or K₂ SiF₆, respectively, c. removing insolubles from the slurry of (b) and collecting the filtrate, d. cooling the filtrate to cause further precipitation of insolubles and then separating the insolubles from the filtrate, e. adding additional sulfuric acid to the filtrate but only if necessary to adjust the Ca/P mole ratio in solution to form greater than 0.5 to about 1.0.
 8. The process of claim 7 wherein the dryer exit gas temperature is from about 200°F. to about 350°F. and the slurry is preconcentrated prior to being spray dried and further including the steps of condensing the water soluble fraction of the gaseous effluent from the spray dryer or otherwise forming an aqueous solution of the water soluble fraction, and then recycling the condensate or solution, without removal of fluorine, for use in acidulating phosphate rock. 